worktree: improve find_worktree() documentation

Do a better job of explaining that find_worktree()'s main purpose is to locate a worktree based upon input from a user which may be some sort of shorthand for identifying a worktree rather than an actual path. For instance, one shorthand a user can use to identify a worktree is by unique path suffix (i.e. given worktrees at paths "foo/bar" and "foo/baz", the latter can be identified simply as "baz"). The actual heuristics find_worktree() uses to select a worktree may be expanded in the future (for instance, one day it may allow worktree selection by <id> of the .git/worktrees/<id>/ administrative directory), thus the documentation does not provide a precise description of how matching is performed, instead leaving it open-ended to allow for future enhancement. While at it, drop mention of the non-NULL requirement of `prefix` since NULL has long been allowed. For instance, prefix_filename() has explicitly allowed NULL since 116fb64e43 (prefix_filename: drop length parameter, 2017-03-20), and find_worktree() itself since e4da43b1f0 (prefix_filename: return newly allocated string, 2017-03-20). Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>